In the experiment diagrammed in Figure, why was it important for the researchers to includ...

In the experiment diagrammed in Figure, why was it important for the researchers to include a tag on the end of the substrate RNAs?

Figure Test-tube selection scheme for identifying ribozymes that can synthesize RNA (a) A large pool of RNA molecules containing random sequences of 220 nucleotides are bound by base-pairing to agarose beads. Copies of a short substrate RNA are added. (b) The substrate binds by base-pairing to the free end of the pool RNAs.(c) If the random portion of a pool RNA’s sequences confers appropriate ribozyme activity, the pool RNA permanently attaches the substrate to its own free end by forming a phosphoester bond. (d) Substrate RNAs not bound by phosophoester bonds are washed away, and the pool RNAs are released from their beads. (e) The pool RNAs are run through an affinity column that catches them by base-pairing with a tag on the free end of the substrate. Only pool RNAs with substrate bound are caught. After Bartel and Szostak (1993).